1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device. In particular, the present invention relates to a method of manufacturing a semiconductor device which is formed over a flexible substrate.
2. Description of the Related Art
In recent years, a technology for semiconductor devices having thin film transistors (hereinafter referred to as TFTs) including semiconductor films formed over insulating substrates has been developed extensively. TFTs are widely used as switching element for operation of a display element, an IC tag, and the like.
A glass substrate, a quartz substrate, and the like have been heretofore used as a substrate for forming TFTs. These substrates have disadvantages in that they are easily broken and are heavy. Therefore, in recent years, a flexible substrate attracts attention in particular. A flexible substrate is thin, light weight, and bendable; therefore, it allows various designs and can be easily formed into flexible shapes. In addition, a flexible substrate is excellent in impact resistance, and can be easily attached or embedded in various goods; accordingly, the flexible substrate can be useful in variety of fields. A plastic substrate can be given as a typical example of a flexible substrate.
Since a manufacturing process of a TFT needs high temperature, in the case of using a plastic substrate, it is necessary to use a plastic having heat resistance which withstands a processing temperature in the manufacturing process. A technique for forming a TFT over a plastic substrate using a low temperature film forming apparatus or laser annealing is disclosed in Patent Document 1 (Patent Document 1: Japanese Published Patent Application No. 2006-100662).
However, in general, a following method (a method including a peeling step and a transferring step) is used: a TFT is provided over a substrate formed of glass, the TFT is peeled off, and the TFT which is peeled off is provided over a substrate formed of a plastic. As a flexible substrate, a film of polyethylene naphthalate (PEN) can be used specifically. A film of poly ethylene terephthalate (PET), a polybutylene naphthalate (PBN), polyimide (PO), polyamnide (PA), or the like may be used as well as a film of polyethylene naphthalate. A technique for forming a TFT over a plastic substrate through a transferring step is disclosed, for example, in Patent Document 2 (Patent Document 2: Japanese Published Patent Application No. 2004-153021).